1. Field of the Invention
The present invention relates to a magnetic filter material useful for a filtering system. More particularly, the present invention relates to a magnetic filter material which is made of a spray-spun, self-bonded, nonwoven structure comprising randomly arranged substantially continuous fibrous material consisting essentially of a polymer and magnetic substance, and which shows excellent filtration efficiency of metal impurities including especially iron components contained in gas or liquid.
2. Description Of The Prior Art
Conventionally, filtering systems refer to those which are installed in the flow path of fluids and act to remove impurities contained in the fluids. In these filtering systems, filter media have been used as an essential element.
A wide variety of materials have been used as such filter media, and these materials include synthetic fibers made of polymers such as polyethylene, polypropylene, etc., glass fibers, cottons, cellulose, iron wire, and pulp. These filter materials are conventionally manufactured in the form of woven fabrics, cartridges, nonwoven fabrics, pleated papers, and so forth.
For example, U.S. Pat. No. 4,361,619 to Forsten discloses a composite filter felt comprised of an intimate blend of PTFE and glass fibers. However, the filter media of this type suffer from the disadvantage that the limited filtration area and porosity thereof reduce the filtration efficiency significantly.
Other types of filter media comprised of glass fibers, synthetic fibers, metal fibers, and so on, in the form of woven or nonwoven fabrics, have been known conventionally, but they have encountered similar problems causing reduced filtration efficiency.
As an attempt to enhance filtration efficiency, a method has been suggested to increase the thickness of filter media. This method could increase the filtration efficiency to some extent. However, if the filter media become larger in thickness, the pressure difference between the inlet and outlet of the media through which filtrates pass increases considerably owing to the media having only a limited capacity to accumulate contaminants, resulting in the filtrates being drawn back and the filter media being destroyed.
Filter media made of pulp materials have been widely used attributing to the large filtration area thereof. However, filter media of this type have a little trap which serves as a space for accumulating the impurities, unlike the woven or nonwoven fabrics made of synthetic fibers. For this reason, it may cause the drawback of a pressure difference, that is, an increase in the difference between the inlet and outlet pressures, and this degrades the filtration efficiency. These problems become serious when the filter material is used as an oil filter medium for a lubricating system. Lubricating oils usually contain an amount of high-molecular polymer additives, such as viscosity improvers and pour point depressants, in addition to impurities. Therefore, when circulating the oils through the filter medium, the pores of the medium may be clogged thereby resulting in an increased pressure difference between the inlet and the outlet of the medium, and in its lower filtration efficiency.
As mentioned above, the filtration efficiency of the filter media depends largely on the porosity thereof. That is, when the filtrate passes through a filter medium, a pressure difference between the inlet and the outlet is created at the outset depending on the porosity of the medium. The difference is likely to increase with the decrease in the porosity.
If the pressure difference becomes higher than a given level, the filtration efficiency is reduced abruptly causing various problems. For example, in the case of a by-pass system in which a portion of the filtrate is by-passed, the impurities are circulated along the flow path within the facility equipped with the system. Meanwhile, in the case of an all-pass system, the impurities contained in the filtrate are accumulated in the filter medium to increase the pressure difference. The greatly increased pressure difference leads to lower filtration efficiency and rupture of the filter media as well as shortness of the cyclic life.
A number of methods have been hitherto suggested in attempt to resolve the above prior art problems.
In an representative example of these methods, an electric charge is applied to a filter medium. For example, U.S. Pat. No. 3,850,600 to Monsted, Jr. discloses a filter medium comprised of synthetic fibers in admixture with resin particles which were electrically charged by a mechanical working. Japanese Laid-Open Patent Publication No. (sho)59-145013 discloses a filter medium which comprises three kinds of polyesters having different melting points and to which an electric charge is applied by heating them under pressure.
Another approach is a method of removing impurities utilizing a magnetic field, as disclosed in U.S. Pat. No. 4,193,866 to Slusarczuk et al. According to this method, suspended impurities are flocculated by adding magnetic ferrite powder, and the flocculated impurities are then separated off in the magnetic field.
By the utilization of these filter medium and systems, filtration efficiency could be more or less increased as compared with the conventional ones. However, the costs of the installation, manufacturing and maintenance thereof are too expensive.
We, the inventors of the present invention, have intensively conducted a wide range of experiments in order to solve the problems encountered in conventional filtering systems. As a result, the inventors have discovered that by spray-spinning-extruding a mixture of a polymer and a magnetic substance to provide self-bonded, nonwoven fibrous structure comprising randomly arranged continuous fibrous material, and forming into a sheet or cylinder, and then magnetizing, the filtration efficiency of the resulting filter material can be enhanced considerably because of the dual function of filtration provided by the ferrous and non-ferrous particles by physical porosity and magnetic attraction, and could accomplish the present invention.